The group, called Best For Britain, will formally launch next week. It eventually aims to endorse a slate of candidates who back its preference for a “meaningful” vote by MPs at the end of the EU negotiation period.

Those challengers deemed most likely to defeat politicians representing the government’s Brexit strategy will be favoured over less competitive third- or fourth-place candidates, and they may also receive support. There will also be specific efforts to mobilise young voters.

“We want to build the biggest tactical voting effort in UK history to ensure that candidates across the country that promise to do what’s best for Britain in the Brexit process get the extra support they need to win,” said Miller.

“If the deal the next government negotiates doesn’t match up to our current terms, MPs should do what’s best for Britain and reject it. We will be asking MPs to pledge to keep an open mind and not be bullied into giving the next government a blank cheque for the final deal.”

Miller, an investment manager whose recent legal challenge forced the government to seek parliamentary approval for invoking article 50, insists the campaign is apolitical in that it could in theory support moderate Tories facing opposition from pro-Brexit Labour MPs.

In practice, the most likely recipients of endorsement and support would be Labour and Liberal Democrat candidates who take a clear stance against a hard Brexit and are poised to threaten Conservative MPs favouring the prime minister’s approach.

May said she was calling the election because “there should be unity here in Westminster” on the issue of Brexit and that the election would prove there could be “no turning back”, a stance some supporters in the media have described as “crushing saboteurs”.

But campaigners like Miller argue that the referendum did not give the government a mandate to pursue Brexit at all costs and that it is undemocratic to ask voters to give the government an unfettered mandate before the terms of any negotiated deal are known. They believe the speed and opportunism of the snap election is designed to snuff out scrutiny and must be met with imaginative new political methods.

“There isn’t time to organise a formal progressive alliance,” Miller told the Guardian when explaining the hastily arranged tactical voting strategy. “We have to do what we can in the time available. We need to re-energise people about the importance of voting tactically.”

The group, which has appointed Eloise Todd as a campaign director, has already held informal talks with political parties about their Brexit policies, but stresses its endorsements will be targeted at individual candidates, some of whom may differ from their leaders.

“We will support different parties and independents, and organisations working in this space,” said Todd, who was executive director of policy for anti-poverty organisation the ONE Campaign. “We are not party political. We stand for democracy in this country and a constitution that respects proper balance of powers and doesn’t railroad the country into an extreme Brexit.”

Miller said she was willing to “bypass the hierarchy of the parties” if necessary because “there is quite a lot of infighting and internal politics”.

In looking to build a coalition of interests that transcends party boundaries, Best For Britain is likely to be one of many remain-leaning groups advocating tactical voting in the weeks to come. Other groups are already mobilising on Facebook and the development echoes US activist groups like the Progressive Change Campaign Committee and the Heritage Foundation, which often endorse a slate of candidates to pursue objectives outside party structures.

But Miller said it was important that the often splintered anti-Brexit movement did not become “like Dad’s Army”. She said: “We need to have a galvanising moment. We need to have a strategy and a structure. Time is not on our side, so we have to put aside egos. It is about being pragmatic.”

By becoming a figurehead for the legal campaign to force a parliamentary vote over article 50, Miller says she was subjected to a torrent of online abuse.

“I’ve been told that ‘as a coloured woman’ I’m not even human – I’m a primate and only a piece of meat and I should be hunted down and killed,” she said in an earlier interview with the Guardian. “I’ve had somebody told me I needed to be ‘the new Jo Cox’.”

But she says she was disappointed that more MPs did not have enough “backbone” to support amendments to the legislation she made possible.

The government has argued that allowing parliament a veto over Brexit at the end of the deal-making process would encourage EU negotiators to try to strike a deliberately unfavourable deal.

Instead, May has hinted that she will allow a vote in 2019 so long as the only alternative is that Britain leaves without a deal if MPs reject it.